The first piece in this two-part series from Hagyard Equine Medical Institute identifies five toxic plants that can invade your horse’s pasture.
by Dr. Gina Tranquillo/Hagyard Equine Medical Institute | May 16, 2017, 1:08 PM EST
The pastures are in bloom. It’s that time of year we all have been anxiously awaiting. Winter is over, and the green is taking over! When you take a walk through your pasture, I’m sure you will find plants that are toxic to your horse. Lots of these plants in general will pose little threat to your horse because they will have no desire to eat them, although one reason horses will eat non-palatable plants is hunger. Let’s take a stroll through the top 10 poisonous plants—five this week and five in next Tuesday’s edition—to watch for in your horse’s pasture, based on a list compiled by the American Association of Equine Practitioners. Up this week: bracken fern, hemlock, water hemlock, tansy ragwort, and Johnson grass/Sudan grass.
Since our equine friends often weigh 1,000 or more pounds, it takes quite a bit of plant intake to affect them clinically, for the most part. However, some plants are a cause for concern with just a little nibble or repeated browsing over weeks to months. Be on the lookout! Poisonous plants are worth recognizing so they can be promptly removed to protect the health of your horse. If you have additional questions, seek the help of your local Equine Extension office as well as your veterinarian.
Bracken Fern (click link for image and additional information)
This grows in woodlands and moist open areas. The entire plant is toxic. It contains thiaminase (an enzyme), which inhibits absorption of thiamin (vitamin B1). Thiamin is necessary for nerve function, and, if there is a thiamin deficit, this can lead to neurological problems. Clinical signs are related to neural dysfunction, including depression, incoordination (ataxia), and blindness. It is said a horse needs to consume three to five percent of its bodyweight over a month to induce clinical deficits. If you catch this before neurological signs are severe and start treatment with your veterinarian, you will have a better outcome.
Hemlock (click link for image and additional information)
This is a weed with clusters of small white flowers. All parts of the plant, leaves, stems, and seeds contain neurotoxins that affect both central and peripheral nervous systems. Four to five pounds is a lethal dose for a horse. Most horses will opt not to graze on it, unless hunger has driven them to. Clinical signs appear within an hour or two after a horse eats the plant. These signs begin with nervousness, tremors, and incoordination. You can even see signs of depression, colic, and decreased heart and respiratory rates. Ultimately, death results from respiratory failure. If your horse has hemlock poisoning, supportive treatment may help in his recovery if he has ingested non-lethal amounts.
Water Hemlock (click link for image and additional information)
Water hemlock, one of the United States’ most toxic plants, often grows in low-lying or marshy areas of meadows, along streams, or by irrigation ditches. All parts of the plant contain a cicutoxin alkaloid that affects the central nervous system. Horses will browse this plant. Less than a pound of leaves and stems can be fatal; however, most of the toxin is concentrated in the root. The toxin primarily affects the brain. Clinical signs may include excess salivation, dilated pupils, nervous attitude, difficulty breathing, seizures, and convulsions, which can lead to death by respiratory paralysis.
Clinical signs can appear within one hour of ingestion, and death typically follows within two to three hours. Supportive care is absolutely necessary before convulsions begin. Horses that survive may have permanent damage to the heart and skeletal muscles. Often, euthanasia is recommended due to rapid onset and severity.
Tansy Ragwort (click link for image and additional information)
This weed produces small, daisy-like, yellow flowers. There are about 70 species that span the United States and many different habitats. These plants contain pyrrolizidine alkaloids and the concentration can vary among species. These plants are especially harmful after being cut, as they are less bitter upon eating—but just as toxic. These alkaloids are cumulative in nature and inhibit cell division, especially in the liver where cells are slowly killed and prevented from regenerating. This liver damage is irreversible. Horses can be the target of chronic exposure over time. Consuming 50-150 lbs. of the plant can result in chronic exposure and disease. Clinical signs of poisoning do not appear until the liver damage is severe; they include photosensitization, jaundice, incoordination, depression, decreased appetite, and weight loss. Euthanasia is recommended when liver failure occurs. The best prevention is to eliminate exposure to the plant.
Johnson Grass/Sudan Grass (click links for images and additional information)
Both plants can grow up to six feet tall and are wild grasses native to southern climates. Leaves and stems contain cyanide compounds. These compounds are metabolized and inhibit the body’s ability to absorb oxygen, resulting in suffocation. Young plants of Johnson grass contain the highest concentrations of the toxin. Clinical signs with acute poisoning are rapid breathing, gasping, frequent urination and defecation, bright red or brick red mucous membranes and progression to convulsions and death. One of the problems in feeding sorghum hay to horses is the potential for permanent damage to nerves of the urinary bladder, which can cause urinary incontinence and hind-end weakness. It is not recommended to feed sorghum hay to horses for prolonged periods of time unless it is from a “cyanide-free” sorghum hybrid plant. Once your horse is affected, there is no effective treatment for nerve damage; however, if the problem is caught early enough, supportive therapy can diminish the effects of cyanide poisoning.
What to do
If you suspect your horse has come in contact with or ingested a poisonous plant, prompt removal of the plant is key. Seek veterinary help immediately, and save a portion of the suspected toxic plant for your veterinarian to identify, if possible. If you can determine the amount of plant ingested by the horse, it is helpful to know, because small amounts of some toxins can be fatal in a short period of time.
Be prepared for your veterinarian to initiate supportive care and stabilize your horse, if possible. This may be followed by referral of your horse to a medical facility for ongoing, 24-hour management if poisoning is advanced. At these facilities, nursing care and veterinary care take place around the clock and your horse can be well monitored and cared for. Remember that even after treatment has begun, poisoning in some cases can cause permanent or irreversible damage.
Prevention, common sense, and good horse management are key. Offering your horse good-quality hay and grain is important. In addition, inspection of your hay at regular intervals is also important. If you have any questions about your feed sources, ask your veterinarians about sampling and inspection of them. And enlist the help of your veterinarian or local county extension office if you have questions about poisonous plants on your farm or in your pastures.
Information in this document has been provided by Hagyard Equine Medical Institute with the help of the American Association of Equine Practitioners.
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